Two inlets and an outlet — Time & Work Aptitude Question
Two inlets and an outlet: Pipe A fills a tank in 12 hours, pipe B in 15 hours, and pipe C empties a full tank in 20… Answer and worked solution on CodeKairo.
- Topic: Time & Work
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 60 seconds
Question
Pipe A fills a tank in 12 hours, pipe B in 15 hours, and pipe C empties a full tank in 20 hours. With all three open, how long does the tank take to fill?
Options
- 8 h
- 10 h
- 12 h
- 15 h
Show the answer and the worked solution
Correct answer: B. 10 h
Worked solution
- Net rate = 1/12 + 1/15 − 1/20 = 5/60 + 4/60 − 3/60 = 6/60 = 1/10.
- The tank fills in 10 hours.
Answer: 10 h.
The idea
Inlets add to the rate, outlets subtract. If the net rate came out negative the tank would never fill, which is worth checking.
Shortcut. Call the tank 60 units: A adds 5 and B 4 an hour, C removes 3, so the net is 6 an hour and the tank fills in 10 hours.
Common trap. 12 h — A's time alone, on the idea that B's inflow roughly cancels C's outflow; B adds 4 units an hour but C removes only 3, so the tank fills sooner.